The storage of fluids in closed cargo tank containers requires pressure relief to allow for changes in pressures in the tanks as the fluids contract and expand through diurnal heating and cooling. Pressure compensation by venting has been especially utilized. Existing differential pressure valves, such as pressure/vacuum relief valves discharge to or suck from a back pressure equal to atmospheric pressure when cargo tank pressure either exceeds or is lower than an acceptable limit. However, as such pressure/vacuum valves typically are set to open at a relatively slight negative pressure differential, so that when a tank ruptures, initial outflow of the cargo will cause the pressure/vacuum valve to open, permitting additional uncontrolled out-flow to occur, as tank pressure is allowed to stabilize with the atmosphere. Manual re-setting of the pressure/vacuum valves during normal operations would be difficult, and would present a safety hazard as an undue lower pressure in the tank could cause the tank to collapse in.
U.S. Pat. No. 4,508,131, issued to DeFrees discloses a valve system for cargo tanks which provides differential pressure relief. A pressure/vacuum valve has a spring bias, requiring manual adjustment to set the valve to a desired required differential pressure to open the valve. There is no discussion of any method or system to bias the pressure/vacuum valve setting to alter the differential pressure requirement between the atmosphere and the tank, necessary to open the valve once the pressure/vacuum set point is established.
U.S. Pat. No. 4,067,352, issued to Causey discloses a pressure/vacuum valve which is designed to maintain a near constant pressure or vacuum in a ship's cargo tank and to provide differential pressure relief. However, this patent requires a system of inert pressurizing gas lines and valves to restore pressure in the cargo tank which was lost through venting to or from the atmosphere. There is no discussion of biasing the pressure/vacuum valve setting to alter the differential pressure requirement between the atmosphere and the tank, necessary to open the valve, after the pressure/vacuum valve set point is established.
U.S. Pat. No. 3,839,982, issued to Martin et al., discloses a vacuum relief valve system designed to open automatically when tank pressure falls slightly below that of the atmospheric pressure. A valve operature restricts air-flow to a velocity below 200 ft/sec. Again, there is no mention of biasing the valve setting to modify the differential pressure at which the valve will open.
Additional prior work includes two (2) related publications by M. Husain: "Advanced Spill Avoidance System For Oil Tankers By Inert Gas Controlled Method Utilizing Vacuum Technique", presented Sep. 28, 1990 to the Marine Technology Society, in Washington D.C., and "Advanced Spill Avoidance System For Unmanned Barges", presented at the Marine Technology Society Conference on Nov. 13, 1991, in New Orleans, La. Both publications disclose a method of modifying existing inert gas pressure maintenance systems to provide underpressure in the cargo tank ullage space, to prevent oil spillage above the rupture line in the case of a tank rupture. A computerized system of inert gas conduits and valves monitors tank pressure levels and initiates a response to equalize the pressures that are inside and outside of a selected cargo tank, to maintain a slight underpressure in the ullage above the cargo. As pressure equalization at a rupture point is attained, a reduction in further oil spillage occurs. Such a system of computerized valves and inert gas conduits is costly, complicated, and requires significant maintenance.
The prior work is limited in the attempts at providing differential pressure relief to a cargo tank that has been ruptured. None of the prior work teaches a system of adding a bias to a pressure/vacuum valve to alter the required differential pressure to open the valve, once the valve has been set. There is therefore a need for such an inexpensive, non-complex system with very few working parts, requiring no manual re-setting of the pressure/vacuum valves and no welding, and which is easily verifiable by local port authorities.